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Under for your interest, the topic, Troubleshooting The
Refrigerent System With Superheat & Subcooling.
It ststes under critical temperture differences.
The low side superheat should be between 20 and 30 de-
grees. A/C manuals i have looked at show adjusting super
heat on fixed orfice systems down to 5 degrees. What am
i missing here?

Originally posted by pipe threader
Under for your interest, the topic, Troubleshooting The
Refrigerent System With Superheat & Subcooling.
It ststes under critical temperture differences.
The low side superheat should be between 20 and 30 de-
grees. A/C manuals i have looked at show adjusting super
heat on fixed orfice systems down to 5 degrees. What am
i missing here?

Fixed orifice superheat will vary with mass flow rate and interior heat load. For example, let's say you have a hot day at 102 degrees, and an interior that the system is able to maintain comfortably at 74 degrees. Since the outdoor ambient is high, the condenser will be running a higher head pressure. This places more pressure on the liquid column feeding the fixed orifice, resulting in a greater pressure differential across the orifice and into the evaporator.

Result? Greater mass flow rate, resulting in more refrigerant entering the evaporator, which gives you a lower superheat. Reduced airflow via dirty filters or air circulation restrictions will only aggravate the condition.

Take a look at Jim Wheeler's charging chart for fixed orifice systems. If you plot out the superheat using his numbers, you can see that with variation in indoor and outdoor conditions, the superheat can range from near zero to pushing 40 degrees. It opened my eyes when I did this, only confirming what I already believed about the fuzziness of fixed restrictor metering devices.

I like shophounds answer. Also for a better understanding of superheat and subcooling check out the thread An Explanation on Superheat & Subcooling, it's also in the For Your Interest section. The best way to charge a system is by using the manufactors charts.